1. Technical Field
This disclosure relates to high power, solid state amplifiers for radio frequency and microwave signals, including pulse and CW radar signals.
2. Description of Related Art
RF and microwave power amplifiers may need to operate under pulsed conditions, such as in civilian and military radar systems. They may need to provide a high peak power to gain surveillance/transmit distance capacity.
Klystron and TWT systems may deliver very high transmit power (megawatts), while occupying relatively small physical volumes. However, these radar systems may provide a mean time between failures (MTBF) of only hundreds of hours. This can be problematic in mission-critical applications, such as in radar on a military ship or in an airport control tower.
Solid state power devices are now capable of delivering 1 kW peak power per device or more. Efforts have been made to achieve solid state high power (>>10 kW) amplifiers (SSHPAs) by combining multiple solid state power devices utilizing multiple stage, conventional passive combining schemes, such as balun, Wilkinson, hybrid, and radial combiners. However, the power level and power density of SSHPAs may not be as great as the rival Klystron.
Reliability can also be a problem. The MTBF of a power amplifier unit (PAU) that includes one or more power devices may be the replica of the failure rate λPAU (per 106 hours) where the power device failure rate λp dominates the total λPAU. According to MIL-HBK-217:λp=λbπTπAπMπQπE  (1)where πT, πA, πM, πQ, and πE are as specified below.
The temperature factor may depend on the junction temperature TJ of a power device (πT=0.1 with TJ=100° C.) as follows:πT=0.1e{−2903(1/(TJ+273)−1/373)}  (2)Application factor may be irrelevant to the specific power device (πA=0.64 with 4% duty cycle as an example):πA=0.06(Duty Cycle %)+0.4  (3)    Matching factor: πM=1.0 for the Input and Output match device    Quality factor: πQ=5.0 for Lower Part    Environmental factor: πE=4.0 for naval sheltered NS as an example
For a power device used in a naval-sheltered environment, assuming 100° C. peak junction temperature with ˜4% duty cycle pulse operation:λp=1.28λb  (4)where the λb base failure rate may be determined by operating frequency F in GHz and peak power P in watts:λb=0.032e{0.354(F)+0.00558(P)}  (5)
Assuming an operating frequency F=1.0 GHz, the following Table 1 presents estimated MTBF of SSHPA utilizing conventional combining architecture with various power levels per device.
Assuming an operating frequency F=1.0 GHz, the following Table 1 presents estimated MTBF of SSHPA utilizing conventional combining architecture with various power levels per device.
TABLE 1Estimated MTBF of Conventionally Combining SSHPAPower Device ConventionallyCombined to form HPAEstimated MTBF (Hr) for HPA*Peak Powerλp (perMTBFincluding power devices and passive components(W)106 Hr)(Hr)5 kW10 kW20 kW40 kW5000.741,350kHr75kHr37.5kHr18.75kHr9.375kHr100012.0882.7kHr9.2kHr4.6kHr2.3kHr1.15kHr20003203.44310Hr69Hr35Hr17.5Hr8.75Hr*The estimation is based on 1.5 times total power device failure rate, as the overall failure rate of SSHPA. The actual MTBF of SSHPA may vary depending upon specific applications, environments, etc.
Thus, further increasing device unit power may not reliably achieve a needed high power. With moderate unit power, the reliability of power under 10 kW may be feasible based on conventional combining architecture. However, it may not be sufficiently reliable for power levels greater than 100 kW. The use of lower power devices in HPA may also create mechanical complications and may not have a needed power density.
This analysis may apply for ultra-long pulse, ultra-high duty cycle, and CW operation power amplifiers with lower output power, because significant junction temperature increases may dramatically decrease the reliability of HPA according to equations 2 and 3.
In general, RF and microwave SSHPAs utilizing conventional combining architecture summing up multiple solid state power devices can face ultra-high power and ultra-high power density challenges in comparison to rival technologies due to limitations in power device availability and/or reliability.
Therefore, the need continues for an RF or microwave amplifier that can handle ultra-high power in excess of 10 kW with high power density and/or ultra-reliability for various signaling applications, including pulsed and continuous wave (CW) operations.